Self-cleaning coating compositions and cooking apparatus coated therewith

ABSTRACT

A self-cleaning coating film is formed using a self-cleaning coating composition including an oxidation, catalyst mixed within a composition including silicone synthetic resin and an organic solvent. The oxidation catalyst includes at least one element selected from a metal such as a noble metal and a metal oxide such as manganese dioxide, copper oxide, iron oxide, nickel oxide, chrome oxide, and the like. The metal is blended with a powder made of alumina, or the like, in the range of about 0.1 to 1.0 weight %. The diameter of the particles of the powder is on the order of about 40 to 300μ. The powder is mixed with the composition in the range of about 5 to 100 weight %. The metal oxide is present in the composition at about 50 weight %. The self-cleaning coating film is formed at about 40 to 300μ thickness. The thus formed self-cleaning coating film is exposed to an elevated temperature of about 300° to 400° Centigrade for about 5 to 10 minutes, whereby the silicone synthetic resin is carbonized to form a film of SiO 2 .

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of copending application Ser. No.229,522, filed on Jan. 30, 1981 (now U.S. Pat. No. 4,374,754) which is acontinuation-in-part of copending application Ser. No. 57,106, filed onJuly 12, 1979 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to self-cleaning coating compositionswhich are disposed on the inner surfaces of a cooking apparatus. Moreparticularly, the present invention relates to self-cleaning coatingcompositions made of silicone synthetic resin and powder containing anoxidation catalyst.

2. Brief Description of the Prior Art

Conventional self-cleaning coating compositions are, for example,disclosed in Japanese Publication No. 47-17832, published May 24, 1972and U.S. Pat. No. 3,460,523, issued Aug. 12, 1969.

The above-mentioned prior art references include organic materials, asbinder or bonding agents for the self-cleaning coating compositions, andan aqueous solvent for dissolving the organic materials. In the JapanesePublication No. 47-17832, the binder is glassy frit materials. U.S. Pat.No. 3,460,523, discloses the use of silicates as the binder in the formof aqueous solutions.

In these prior art references, a disadvantage is that the surfaces onwhich the conventional self-cleaning coating compositions are formedmust be completely and specially treated to prevent the self-cleaningcoating compositions from being shed therefrom.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provideimproved self-cleaning coating compositions.

It is another object of the present invention to provide improvedself-cleaning coating compositions which may be easily formed onsurfaces of any cooking apparatus.

It is a further object of the present invention to provide improvedself-cleaning coating compositions which adhere to surfaces of anycooking apparatus, where a desired strength, hardness, abrasionresistance and toughness of the coating film are obtained.

It is another object of the present invention to provide an improvedself-cleaning coating composition for application to a surface whichincludes an organic solvent whereby the solvent functions as a solventfor the silicone synthetic resin and whereby the solvent exerts acleaning action on said substrate.

It is another object of the present invention to provide a coatingcomposition which may be coated onto a variety of heat stable substratessuch as metal plates, and interior oven surfaces.

The silicone synthetic resin self-cleaning coating composition containsat least three ingredients, i.e., a silicone synthetic resin, a heatstable oxidation catalyst and a solvent. The coating composition usuallyalso includes carrier particles for the oxidation catalyst. However, thecoating composition may also include various other additives such asbentonite or the like, and/or various other assistants such as powderedsilica filler or the like.

Various types of silicone synthetic resins such as straight siliconesynthetic resins, silicone synthetic resin intermediate products andmodified silicone synthetic resins are useful in accordance with theteachings of the present invention. The silicone synthetic resins mustbe able to be formulated into a composition which can be easily coatedonto a surface. The silicone synthetic resins must also be able toeffectively bind oxidation catalyst particles and must be able to form ahard stable carbonized coating after exposure to carbonizing conditions.

The straight or unmodified silicone synthetic resins are siliconesynthetic resins which are not polymerized, blended or mixed with anyother non-silicone synthetic resin. This is a type of primary polymer.These resins may also be referred to as pure and non-blended siliconesynthetic resins. These resins have various types of organic groups suchas lower alkyl, preferably methyl, or aryl, preferably phenyl, groupseach of which is connected to a silicone atom. Specific examples aremethyl synthetic silicone resins where the organic group connected toeach of silicone atoms is primarily methyl or methyl only andphenylmethyl synthetic silicone resins where the organic groupsconnected to each of silicone resins are methyl or phenyl. The lattertype of silicone synthetic resin may comprise recurring units of thefollowing formula: ##STR1## The silicone intermediate products may beidentified as low molecular weight silicone synthetic resins havingfunctional groups such as a lower alkoxyl group, such as methoxyl, or ahydroxyl group connected to the end of a molecule. These siliconeintermediate products usually have a molecular weight between about1,000 and about 1,500. The silicone intermediate products may be blendedwith non-silicone polymers such as polyesters, polyolefins, epoxy,alkyd, acryl, urethane, melanine, phenol, or the like. The siliconeintermediate products may also be blended with other non-siliconeintermediate products such as low molecular weight polymers, prepolymersand the like.

The modified silicone synthetic resins may be a straight siliconesynthetic resin which is blended with another polymer such as apolyester, a polyolefin, an epoxy, an alkyd, ester, acryl, urethane,melanine, phenol, or the like.

These modified silicone synthetic resins may also be copolymers obtainedby copolymerizing a silicone synthetic resin monomer or a siliconeintermediate product with another monomer such as an epoxy, an alkyd,ester, acryl, urethane, melanine, phenol, or the like. The weight of thenon-silicone component of the silicone synthetic resin should not exceedabout 1.8 times the weight of the silicone component of the resin.Modified silicone synthetic resins which contain at least 50%,preferably between about 50 and 90%, of the silicone synthetic resincomponent are preferred.

One example of a modified silicone synthetic resin is a silicone resincopolymerized with epoxy which can be represented as follows: ##STR2##wherein m is an integer between about 2 and 80 and l is between about 10and 20, and n is between about 0 and 12.

Blending methods such as the cold blending method which is normally usedfor preparing only the blend of the straight synthetic resin and theother synthetic resin or the heat blending method which is normally usedfor effecting a copolymerization reaction between the straight syntheticresin or silicone intermediate product and other synthetic resin may beused. Any one of the above resins or mixtures thereof may be used as asolution in a suitable organic solvent when applied as a self-cleaningcoating material of the present invention. Many types of silicone resinsare examined to determine whether they can be used as a binder materialfor bonding oxidation catalyst particles to each other. As a result, itis found that most of them can satisfy this requirement.

When the resin blended or copolymerized with the straight silicone resindoes not have in its main chain a group such as "--TiO--" "--AlO--" or"--SiO₂ --" like silicone resin copolymerized with epoxy or siliconeresin copolymerized with alkyds, it is preferable that the weight of theresin blended should not exceed about 1.8 times the weight ofstraight-silicone resin.

Various types of organic solvents are useful in accordance with theteachings of the present invention. The solvents are preferablynon-aqueous solutions. Useful solvents include alcohols such asmethanol, ethanol, iso-propanol butanol, diacetone alcohol, ethers suchas diethyl ether, dioxane, and butyl cellosolve, aromatic solvents suchas benzene, toluene, xylene, ketones such as acetone methyl ethyl ketone(MEK), chloroform, carbontetrachloride, or the like.

The preferred solvents are toluene, xylene, diacetone alcohol, butylcellosolve or the like. While the above solvents have been given asexamples of solvents which might be useful, any solvents which caneffectively disperse the various components of the silicone syntheticresin composition to form a coatable composition may be used. Thesilicone synthetic resin composition contains between about 50 and 70weight % of the organic solvent.

The oxidation catalyst is any heat stable oxidation catalyst which caneffectively oxidize fats, oils, grease and other materials which fallonto the self-cleaning coating film during cooking. The oxidationcatalyst may be any compound which meets the above requirements such asa metal or a metal oxide or mixtures thereof. Noble metals such aspalladium and platinum are preferred. Examples of useful metals arecobalt, manganese, iron, copper, nickel, or the like, with a surfacearea in excess of 0.1 square meter per gram. Oxides of the above metalsare also useful. Examples of useful metal oxides are manganese dioxide,copper oxide, iron oxide, nickel oxide, chrome oxide, titanium oxide,cobalt oxide, or the like.

The metals and metal oxides are preferably in the form of particleshaving an average particle diameter between about 10 and 200μ,preferably between about 50 and 70μ. The oxidation catalyst is presentin the silicone synthetic resin composition in an amount between about0.1 and 50 weight %, preferably between about 0.1 and 30 weight %.

The coating composition will also usually include carrier particles forthe oxidation catalyst particles. These carrier particles are preferablymade of a heat stable powdery material such as alumina(Al₂ O₃),cordierite(2MgO.2Al₂ O₃.5SiO₂), mullite (3Al₂ O₃.2SiO₂) etc. Thediameter of these carrier particles is on the order of about 40-300μ,preferably 40-200μ. The powder is utilized for expanding the surfacearea over which the oxidation catalyst particles are dispersed. Thecarrier particles should be present in the silicone synthetic resincoating composition in an amount up to about 70% by weight, preferablybetween about 30 and 70 weight %, most preferably between about 50 and60 weight %.

To achieve the above objects, pursuant to an embodiment of the presentinvention, a self-cleaning coating film is formed using a self-cleaningcoating composition including a heat-stable oxidation catalyst mixedwithin a composition including silicone synthetic resin and an organicsolvent. The oxidation catalyst includes at least one member selectedfrom a metal such as a noble metal, a metal oxide such as manganesedioxide, copper oxide, iron oxide, nickel oxide, chrome oxide, and thelike. The metal or metal oxide is blended with powder made of aluminaetc. in the range of about 0.1 to 1.0 weight % to form a mixed powder.The diameter of the particles in the powder is in the order of about40-300μ. The mixed powder is mingled within the composition in the rangeof about 5 to 95 weight %, preferably between about 50 and 95 weight %.

The metal oxide is combined into the composition in about 50 weight %.The self-cleaning coating film is formed about 40 to 300μ in thickness.

The thus formed self-cleaning coating film is exposed to an elevatedtemperature of about 300° to 400° Centigrade for about 5 to 10 minutes,whereby the silicone synthetic resin is carbonized to form a film ofSiO₂.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. It should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

DETAILED DESCRIPTION OF THE INVENTION

Conventional painting materials in which an appropriate oxidationcatalyst of the present invention is mixed will be described first.These painting materials have thermostable components which arethermostable up to 600° Centigrade. These painting materials are in theform of organic solutions as summarized in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                          Weight %                                                    Components          Sample 1 Sample 2                                         ______________________________________                                        Binder     Silicone     40       38.1                                         or Bond-   Synthetic                                                          ing Mater- Resin                                                              ial                                                                           Pigment    Black Pigment                                                                               25*.sup.1                                                                             24.5*.sup.2                                                                   +10.3*.sup.3                                 Additive   Assistant*.sup.4                                                                           25       18.7                                         Solvent    Organic*.sup.5                                                                             10       8.4*.sup.6                                              Solvent                                                            ______________________________________                                         *.sup.1 A composition of powder made of at least one metal material           selected from Co, Mn, Fe, Cu, Ni and the like and powder made of metal        oxide materials                                                               *.sup.2 A black powder made of metal oxide                                    *.sup.3 Mica                                                                  *.sup.4 Compositions of a kind of silicate in the form of organic solvent     *.sup.5 Agents selected from ketones, xylole, toluene alcohol and the lik     *.sup.6 Agents made of xylone and the like                               

The various components of the composition, except for the organicsolvent, have heat-stable properties.

The following examinations were carried out to select useful oxidationcatalysts added to the painting materials of Table 1.

Preliminary Examinations

Mixed oil of 60 weight % and one of the following oxidation catalysts at40 weight % were kneaded together. The mixed oil includes 10% butter,20% corn oil, 10% fish oil, 20% beef oil, 20% horsebean oil, 20% soybeanoil, all units being weight %. The tested oxidation catalysts werepreliminary heated at about 500° centigrade for about an hour.

Compositions made of the mixed oil and the tested oxidation catalystswere coated on a heat-stable glass substrate at a thickness of about 30μm, which were heated at about 250° centigrade for about one hour.

Standard of Judgement

After the heating at about 250° centigrade for about one hour, theevaporation ratio of the mixed oil was determined. The evaporation ratiois represented by the equation: ##EQU1##

It can be considered that the data of the evaporation ratio over 100%are derived from the facts that oxygen and the like included within theoxidation catalysts was evaporated. The data having a considerably smallevaporation ratio can be thought due to a small degree of evaporatedmixed oil.

Results of Preliminary Examination

The following evaporation ratios were obtained for the respectiveoxidation catalysts to determine whether they are appropriate for thepresent invention.

                  TABLE 2                                                         ______________________________________                                                          Evaporation Ratio                                           Tested Oxidation Catalysts                                                                      %                                                           ______________________________________                                        Ziroconium        33.2                                                        Titanium oxide    23.8                                                        vanadium          21.3                                                        chromium          12.4                                                        manganese dioxide 126.5                                                       nickel oxide      123.7                                                       iron oxide        30.3                                                        cobalt oxide      10.8                                                        tungsten oxide    18.4                                                        molybdenum oxide  12.3                                                        copper oxide      14.8                                                        zinc oxide        12.6                                                        0.5% palladium    81.4                                                        0.2% platinum     86.9                                                        ______________________________________                                    

According to the results of the preliminary examination, the oxidationcatalysts of the present invention should be selected from a noble metalmaterial such as palladium, platinum and the like, and an oxided metalmaterial such as manganese oxide, nickel oxide and the like. The noblemetals can be utilized in a simple body or in a mixed form with eachother and/or with the metal oxide material. The metal oxide materialscan also be used in a mixed form with each other.

The following examples are presented in detail to show the products,processes and uses of the present invention.

EXAMPLE 1

The painting materials as shown in Table 1 are utilized for thisexample. The painting materials to be applied include solid bodies ofabout 50 to 80 weight % which is represented by an amount of the residueafter heating at about 250° centigrade for about 3 hours. The siliconesynthetic resin and the organic solvent can be dissipated byevaporation.

Within the painting materials, there is uniformly mixed a volume ofpowder comprising carrier particles of about 40 to 300μ diameter. Thepowder permeates or holds by coating a material of platinum in about 0.1to 1.0 weight %.

It is preferable that the powder containing the oxidation catalyst bemingled with the painting material at 1:2 in the ratio of weight. Otherratios such as 1:1, 1:3 and 1:4 of the powder within the paintingmaterials are also acceptable. Besides Pt, a metal material such aspalladium, ferrite and like is acceptable for the oxidation catalyst.

Self-cleaning coating compositions of the present invention are obtainedas described above. Coating processes for the present self-cleaningcoating compositions are specified as follows:

A substrate to be coated with the present self-cleaning coatingcompositions is initially prepared. The substrate is preferably a platemade of cold rolled steel plate (SPCC steel plate), a steel plate gildedby aluminum or zinc metal, a stainless steel plate, iron casting and thelike. The substrate is defatted with a suitable organic solvent or bymeans of emulsion methods with the aid of weak alkaline or neutralreagents. The substrate is washed and then dried after the oilextraction or defatting.

Thereafter, the present self-cleaning coating compositions which areadjusted with a suitable organic solvent such as ketone, xylole,toluene, alcohol, and the like are coated on the substrate. A coatedfilm from about 40 to 300μ thickness, preferably about 150 to 250μthickness, is obtained by a procedure wherein the spraying method, thedrying process and the baking process are each carried out only once.

Another coated film from about 100 to 200μ thickness is derived by aprocedure wherein a sequence of the spray method and then the dryingprocess is repeated twice, and thereafter the baking process isperformed.

It is required that the oxidation catalyst particles be exposed orappear from the painting material, especially, the silicone syntheticresin in an amount enough to be effective.

To this end, the silicone synthetic resin is baked for carbonizationpurposes by disposing the coated film within a furnace heated at about300° to 400° centigrade for about 5 to 10 minutes whereby the siliconesynthetic resin is converted into a SiO₂ film. The coated film thusobtained is porous, has a good reliability in obtaining desiredstrength, hardness, adhesion and is tough enough to provide goodcatalytic activity. There is no problem, even with insufficientdefatting because of the utilization of the organic solvent.

The results in Table 3 below were obtained by a test wherein salad oilwas sprinkled over a film made of the present self-cleaning coatingcompositions which was coated on surfaces of a cooking apparatus, forexample, a gas oven. The sprinkled salad oil was dried and fixed atabout 150° to 200° centigrade. Self-cleaning properties were examined insuch a manner and the gas oven was intermittently operated to provideheat of a temperature of about 300° to 400° centigrade for about 20hours.

An example of a coated film made of the present self-cleaning coatingcompositions formed on the surfaces of a gas oven is as follows, a steelplate gilded by aluminum layers is utilized for oven surfaces of a gasoven. On the aluminum layer, there is formed a coated film made of thepresent self-cleaning coating compositions preferably in the range fromabout 40 to 300μ thickness.

                  TABLE 3                                                         ______________________________________                                        Experiment                                                                            Self-Cleaning Coating Compositions                                                                   Evaluation                                     ______________________________________                                        1       Powder made of alumina comprising                                                                    ⊚˜  ○                      particlcs of 74 to 297μ diameter                                           containing 0.2% Pt                                                            [Mixed Ratio]                                                                 Painting Materials including the                                              solid bodies of about 50 to 60                                                weight %:                                                                     Powder                                                                        = 2:1                                                                         (The mixed ratio is common to all                                             the experiment except for Experiment                                          4)                                                                    2       Powder made of alumina comprising                                                                    ⊚˜  ○                      particles of 44μ to 74μ diameter                                        containing 0.2% Pt                                                    3       Powder made of alumina comprising                                                                    ⊚                                       particles of 40μ diameter containing                                       0.5% Pt                                                               4       Only the painting materials listed                                                                   X                                                      in Examples 1 and 2 of Table 1                                        5       Powder made of ferrite comprising                                                                    Δ˜  ○                               particles of 74 to 297μ diameter                                           containing metal oxide                                                6       Powder made of ferrite comprising                                                                    Δ˜   ○                              particles of 44 to 74μ diameter                                            containing metal oxide                                                ______________________________________                                         ⊚: very superior                                                ○ : considerable efficiency                                           Δ: a little efficiency                                                  X:no efficiency                                                          

EXAMPLE 2

The painting material of Table 1 for containing the oxidation catalystpowder of the present invention is replaced by that of Table 4. The kindof the compositions of Table 4 is not changed, only the ratio of thecompositions utilized is modified.

                  TABLE 4                                                         ______________________________________                                        Compositions          Weight %                                                ______________________________________                                        Binder      Silicone Synthetic                                                                          40                                                              Resin                                                             Pigment     Black Pigment 25                                                              (common to that of                                                            sample 1 of Table 1)                                                          Mica          10                                                  Addition    Assistant     15                                                  Solvent     Organic Solvent                                                                             10                                                  ______________________________________                                    

The painting materials shown in Table 4 include a ratio of the solidbodies of about 60 to 80 weight %. Within the painting materials of thistype, there is uniformly combined a certain amount of the powdercomprising the particles in the range of about (60±20)μ diameter. Thepowder is made of the same aluminum compound as in Example 1. The powdercontains, or carries by means of a coating, a platinum material in theratio of about 0.1 to 0.5 weight %. An example of the presentself-cleaning coating materials in this example is thus obtained.

The coating process for the self-cleaning coating materials of this typeis repeated in the same manner as Example 1. The coated film by theself-cleaning coating materials of this type is formed in the range ofabout 40 to 120μ thickness through the use of the process wherein eachof the spray method, the drying process, and the baking process thereforare performed only once. It is preferable that the coated film bemounted on the surface of a substrate in the range of about 40 to 300μthickness.

The following samples are obtained using a powder made of alumina, theparticles of which being about 40μ diameter. The powder contains 0.5% Ptor 0.2% Pt in the range of about 0.1 to 0.5 weight %. The paintingmaterials used contain a ratio of the solid bodies of about 60 weight %.The powder is blended into the painting materials in the range of about2, 5, 8, 12, 15 and 20 weight %, respectively.

                  TABLE 5                                                         ______________________________________                                                                   Alumina                                                         Alumina Powder                                                                              Powder                                             Contents (weight %)                                                                        containing 0.2% Pt                                                                          containing 0.55% Pt                                ______________________________________                                        2            Δ-x     Δ                                            5            Δ-  ○                                                                          ○                                           8            ○      ⊚                                   12           ⊚                                                                            ⊚                                   15           ⊚                                                                            ⊚                                   20           Δ-  ○                                                                          Δ-  ○                                 nil          x             x                                                  ______________________________________                                         ⊚: very superior                                                ○ : enough efficiency                                                 Δ: a little efficiency                                                  x: no efficiency                                                         

The results listed in Table 5 were obtained by an examination wherein anamount of salad oil, for example, 3 cc was sprinkled over a surface ofthe gas oven on which the present self-cleaning coating film was formed.The gas oven was operated to provide heat of about 250° to 300°centigrade for about 8 hours, which is a usual temperature for gasovens. The above stated data was obtained after the operation of the gasoven in such a manner.

As apparently noted from Table 5, the powder containing Pt as theoxidation catalyst should be mixed with the painting material in therange of about 5 to 20 weight %. This kind of the powder contributes tothe enhancement of the self-cleaning characteristics and the persistenceof the coating film.

Another kind of the powder having Pt below 5 weight % is notcontributive to enhancing the self-cleaning characteristics althoughthere is the persistence.

In addition to Pt, a noble metal material such as palladium (Pd) ispreferable for the oxidation catalyst of the present invention, thepowder such as alumina containing the noble metal material beingcontained within the painting material in the order of about 5 to 20weight %.

EXAMPLE 3

In this instance, the painting materials are utilized in the samecompositions as in Table 1 and Table 4. Various oxidation catalysts areblended within the painting materials as summarized in Table 6. Theevaporation ratio (%) defined above is determined by sprinkling a volumeof the salad oil, say, about 5 cc over the self-cleaning coating film ofthe present invention. The self-cleaning coating film mounting thevolume of the salad oil is heated at about 250° centigrade for about 1hour.

                  TABLE 6                                                         ______________________________________                                        Mixing ratio of a                                                             material containing        Evapora-                                           oxidation catalyst to      tion                                               painting material          Ratio    Evalua-                                   (weight %)  The material   (%)      tion                                      ______________________________________                                        Painting material:                                                                        Alumina powder con-                                                                          42       ⊚                          the material                                                                              taining 0.2%                                                      (common in  Pt                                                                hereinafter)                                                                  = 2:1                                                                         3:1         The same       36       o                                         2:1         Alumina powder con-                                                                          44       ⊚                                      taining 0.5% Pt                                                   2:1         Alumina powder con-                                                                          32       o                                                     taining 0.5% Pd                                                   2:1         manganese dioxide                                                                            38       o                                         2:1         nickel oxide   33       o                                         2:1         copper oxide   18       x                                         2:1         iron oxide     24       Δ                                   2:1         manganese dioxide and                                                                        44       ⊚                                      nickel oxide (the same                                                        amount in weight as                                                           common hereinafter)                                               2:1         manganese dioxide and                                                                        33       o                                                     copper oxide                                                      2:1         manganese dioxide and                                                                        36       o                                                     iron oxide                                                        2:1         copper oxide and iron                                                                        18       x                                                     oxide                                                             ______________________________________                                         ⊚: very superior (evaporation ratio above 40%)                  ○ : enough efficiency (evaporation ratio between 30 and 40%)          Δ: little efficiency (evaporation ratio between 20 and 30%)             x: efficiency (evaporation ratio below 20%)                              

The particles of the alumina powder used have the same diameter asdescribed above. The self-cleaning coating films are formed in about200μ thickness.

As noted from Table 6, a certain metal oxide such as manganese dioxide,nickel oxide, and the like is appropriate as the oxidation catalyst andthere is no need to mix a powder such as alumina with the same. Chromeoxide is also available. Compositions combined with at least two kindsof the metal oxide are also useful in the present invention.

The present self-cleaning coating compositions as specified in Examples1 to 3 can be applied to any cooking apparatus such as an electric oven,a gas oven, a microwave oven, especially, for providing elevatedtemperatures, a combined cooking apparatus including at least two kindsof heating sources, and the like. As an example, a combined microwaveoven and electric oven, the surfaces of which carry the self-cleaningcoating film of the present invention, may be utilized.

The combined cooking apparatus comprises a housing containing amagnetron, interior surfaces, an upper heater and a lower heater.

There may be further provided a wave guide, a cover for the wave guide,a bottom wall, a plate for carrying a foodstuff, and an oven door.

The silicone synthetic resin to be used for the present inventionincludes at least one member selected from straight silicone resins,silicone resins copolymerized with alkyds, silicone resins copolymerizedwith epoxy, and silicone resins copolymerized with acrylics, or the likeaccording to the teaching of this application.

The preparation and properties of various coating materials aredescribed below:

                  TABLE 7                                                         ______________________________________                                        COATING MATERIAL                                                              Components Example           Weight %                                         ______________________________________                                        binder     silicone synthetic resin                                                                        15                                                          copolymerized with epoxy*                                          catalyst   MnO.sub.2         30                                               assistant  powered silica    30                                               additive   Bentonite         10                                               organic    Butyl cellosolve  15                                               solvent    Xylene                                                                        Diacetone alcohol                                                  ______________________________________                                         *Any of ES1001 and ES1004 (ShinEtsu Chemical Co., Ltd.) and TSR194            (Toshiba Silicone,Ltd.)                                                  

While the ratio of the examples mixed is maintained, each of theexamples of binder and other agents can be replaced by any suitablematerial according to the teaching of the present invention. Thefollowing data are obtained under such a modification.

The respective coating materials are prepared by such a replacement.Each of them is coated on a suitable plate, preferably, a steel plategilded with an aluminum film of about 200 to about 250 μm in thickness.Pre-baking of this film is carried out at about 80° centigrade for about15 minutes. Further, the film is baked at about 400° centigrade forabout 15 minutes.

2. Examination Method

2-1. Hardness Examination: The peripheral edge of a copper coin isstressed under a load of about 1 kg for a travel of about 0.5 m directlyon the surface of the above described film.

Judgement

○o : no scratch

O: scratch less than about 5% of the travel

Δ: scratch less than about 30% of the travel

x: scratch more than about 30% of the travel.

2-2. Adherence Examination: On the surface of the coated film 36 squareshaving a side of about 5 mm are formed in a checker format by a cutter.The depth of respective grooves formed reach the surface of the steelplate from the surface of the coated film. Broken pieces of the film atthe groove crossing points are measured with the eye.

Judgement

○o : no pieces

O: a small diameter and a small amount of broken pieces

Δ: a small diameter and a large amount of broken pieces

x: a large diameter and a large amount of broken pieces.

2-3. Examination of Alkali Resistivity: a 3% NaOH solution is heated upto about 40° centigrade and the plate coated with the resin is soakedfor about 3 hours. Thereafter, the sample is removed from this solutionand the properties thereof judged.

Judgement

○o : no change

O: a slight discoloration

Δ: a small chalking on the surface of the film

x: removal of the film on the substrate.

3. Result of Examination

3-1. Straight synthetic silicone resin:

H: hardness examination

Ad: adherence examination

A: examination of alkali resistivity.

                  TABLE 8                                                         ______________________________________                                        (1) Silicone Resins                                                                    Example of                                                           Kind of  resin         manufacturer                                           resin    (commercial name)                                                                           of resin    H   Ad  A                                  ______________________________________                                        methyl   KR-220        Shin-Etsu   ⊚                                                                  ○                                                                          Δ                            silicone               Chemical Co.,                                                                 Ltd.                                                   methyl   XR-3370       Toshiba     ⊚                                                                  ○                                                                          Δ                            silicone               Silicone Co.,                                                                 Ltd.                                                   methyl   RHODORSIL     Dainippon Ink                                                                             ⊚                                                                  ○                                                                          Δ                            silicone RESIN 10336   & Chemicals,                                                                              ⊚                                                                  ○                                                                          Δ                                                   Inc.                                                   phenylmethyl                                                                           SH-804        Toray silicone                                                                            ⊚                                                                  ○                                                                          ○                           silicone               Company, Ltd.                                          phenylmethyl                                                                           TSR-117       Toshiba     ○                                                                          ⊚                                                                  ⊚                   silicone               Silicone Co.,                                                                 Ltd.                                                   phenylmethyl                                                                           TSR-145       Toshiba     ⊚                                                                  ○                                                                          ○                           silicone               Silicone Co.,                                                                 Ltd.                                                   phenylmethyl                                                                           KR-282        Shin-Etsu   ○                                                                          ⊚                                                                  ⊚                   silicone               Chemical Co.,                                                                 Ltd.                                                   phenylmethyl                                                                           RHODORSIL     Dainippon Ink                                                                             ○                                                                          ○                                                                          ○                           silicone RESIN 6405    & Chemicals                                                                   Inc.                                                   phenylmethyl                                                                           BAYSILONE     Bayer       ○                                                                          ○                                                                          ○                           silicone RESIN P-150 K Aktiengesell-                                                                             ○                                                                          ○                                                                          ○                                                  schaft                                                 ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        (2) Silicone Intermediate Product                                             Example of resin                                                                           Manufacturer                                                     (commercial name)                                                                          of resin        H     Ad    A                                    ______________________________________                                        TSR-160      Toshiba Silicone Co.,                                                                         ○                                                                            ○                                                                            ○                                          Ltd.                                                             DR-216       Shin-Etsu Chemical                                                                            ○                                                                            ○                                                                            ○                                          Co., Ltd.                                                        ______________________________________                                    

                                      TABLE 10                                    __________________________________________________________________________    (3) Modified Silicone Synethetic Resin                                                                      Sili-                                                                             Or-                                                 Example of            cone                                                                              ganic                                               resin                 resin                                                                             resin                                       Kind of (commercial                                                                          Manufacture    weight                                                                            weight                                      resin   name)  of resin H Ad                                                                              A ratio                                                                             ratio                                       __________________________________________________________________________    siliconeresin                                                                         TSR-180                                                                              Toshiba  Δ                                                                         ⊚                                                                Δ                                                                         1   1                                           copolymerized  Silicone Co.,                                                  with alkyds    Ltd.                                                           silicone resin                                                                        SA-4 Shin-Etsu                                                                       Δ  ⊚                                                                Δ                                                                         1 1                                               copolymerized  Chemical Co.,                                                  with alkyds    Ltd.                                                           silicone resin                                                                        TSR-187                                                                              Toshiba  Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Silicone Co.,                                                  with polyester Ltd.                                                           silicone resin                                                                        AY-42-001                                                                            Toray    Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Silicone                                                       with polyester Company, Ltd.                                                  silicone resin                                                                        UD-125 Bayer    Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Aktiengesellschaft                                                                     Δ                                                                         ○                                                                        Δ                                                                         1   1                                           with polyester                                                                silicone resin                                                                        KR-3093                                                                              Shin-Etsu                                                                              Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Chemical Co.,                                                  with acryl     Ltd.                                                           silicone resin                                                                        TSR-170                                                                              Toshiba  Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Silicone Co.,                                                  with acryl     Ltd.                                                           silicone resin                                                                        SE-653 Mitsubishi                                                                             Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Rayon Co.,                                                     with acryl     Ltd.                                                           silicone resin                                                                        TSR-175                                                                              Toshiba  Δ                                                                         ○                                                                        Δ                                                                         1   1                                           copolymerized  Silicone,                                                      with urethane  Ltd                                                            silicone resin                                                                        ES-1001                                                                              Shin-Etsu                                                                              ○                                                                        ○                                                                        ○                                                                        4   6                                           copolymerized  Chemical Co.,                                                  with epoxy     Ltd.                                                           silicone resin                                                                        ES-1004                                                                              Shin-Etsu                                                                              ○                                                                        ○                                                                        ○                                                                        4   6                                           copolymerized  Chemical Co.,                                                  with epoxy     Ltd.                                                           silicone resin                                                                        TSR-194                                                                              Toshiba  ○                                                                        ○                                                                        ○                                                                        4   6                                           copolymerized  Silicone,                                                      with epoxy     Ltd.                                                           silicone resin                                                                        TSR-145                                                                              Toshiba 80                                                                             ○                                                                        ○                                                                        Δ                                                                         8   2                                           copolymerized  Silicone, w %                                                  with melanine  Ltd.                                                                   L-109-65                                                                             Dainippon 20                                                                  Ink & w %                                                                     Chemicals                                                                     Inc.                                                           silicone resin                                                                        TSR-160                                                                              Toshiba 70                                                                             ○                                                                        ○                                                                        Δ                                                                         7   3                                           copolymerized  Silicone w %                                                   with melanine  Co., Ltd.                                                              YUBAN 220                                                                            Mitsui Toatsu                                                                 Chemicals, 30 w %                                                             Inc.                                                           silicone resin                                                                        TSR-145                                                                              Toshiba 60                                                                             ○                                                                        ⊚                                                                ○                                                                        6   4                                           copolymerized  Silicone w %                                                   with phenol    Co., Ltd.                                                              CKM-908                                                                              Showa 40 w %                                                                  Union Co.,                                                                    Ltd.                                                           silicone resin                                                                        TSR-145                                                                              Toshiba 40 w %                                                                         ○                                                                        ⊚                                                                ○                                                                        4   6                                           copolymerized  Silicone                                                       with phenol    Co. Ltd.                                                               phenol Dainichiseika                                                          resin  Color &                                                                       Chemicals                                                                     Mfg. Co., 60 w %                                                              Ltd.                                                           __________________________________________________________________________

A temperature at which resin material carbonizes by baking is withinabout 320° to about 500° C., more preferably within about 370° to about420° C. In connection with heat stability, a temperature at which thecoated resin film is resistant is below several tens degree centigrade,more preferably, about 30° C. than the temperature for the carbonizationbaking. The assistant functions only to enlarge the total mass of theresin material. Any suitable material of the assistant except poweredsilica may be selected.

While only certain embodiments of the present invention have beendescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thespirit and scope of the invention as claimed.

What is claimed is:
 1. A cooking apparatus including an oven wall onwhich a self-cleaning coated film which catalytically oxidizes andremoves fats and oils deposited thereon is formed, said self-cleaningcoated film being formed from a self-cleaning coating composition,comprising:(a) at least one silicone synthetic resin binder selectedfrom the group consisting of straight silicone resins, silicone resinscopolymerized with alkyds, silicone resins copolymerized with epoxy, andsilicone resins copolymerized with acrylics; (b) an organic solvent; and(c) 0.1 to 30% by weight of a thermostable oxidation catalyst mingledwithin a composition of said silicone synthetic resin and said organicsolvent.
 2. The cooking apparatus according to claim 1, wherein theoxidation catalyst includes at least one material selected from a metalmaterial and a metal oxide.
 3. The cooking apparatus according to claim1, wherein the self-cleaning coating film is formed in the range ofabout 40 to 300 microns thickness.
 4. The cooking apparatus according toclaim 3, wherein the self-cleaning coating film is exposed to anelevated temperature of about 300° to 400° Centigrade for about 5 to 10minutes so that the silicone resin is carbonized to form a film of SiO₂.5. The cooking apparatus according to claim 1, wherein the metalmaterial includes noble metal.
 6. The cooking apparatus according toclaim 5, wherein the noble metal is Pt or Pd.
 7. The cooking apparatusaccording to claim 2, wherein the metal oxide is selected from the groupconsisting of manganese dioxide, copper oxide, iron oxide, nickle oxide,chrome oxide, and the like.
 8. The cooking apparatus according to claim7, wherein the metal oxide is manganese oxide or nickle oxide.
 9. Acombination cooking oven comprising:(a) an oven wall defining an ovencavity; (b) microwave generation heating means for supplying said ovencavity with microwave energy; (c) second heating means for heatingfoodstuff disposed in said oven cavity by heat energy generated by saidsecond heating means; and (d) a self cleaning coating film whichcatalytically oxidizes and removes fats and oils deposited thereonformed on the inner surface of said oven wall, said self-cleaningcoating film comprising: at least one silicone synthetic resin binderselected from the group consisting of straight silicone resins, siliconeresins copolymerized with alkyds, silicone resins copolymerized withepoxy, and silicone resins copolymerized with acrylics; an organicsolvent; and a thermostable oxidation catalyst mingled within saidsilicone synthetic resin and said organic solvent.
 10. A cookingapparatus including an oven wall on which a self-cleaning coated filmwhich catalytically oxidizes and removes fats and oils deposited thereonis formed, said self-cleaning coated film being formed from a coatingcomposition comprising:a silicone synthetic resin selected from thegroup consisting of straight silicone synthetic resin, siliconeintermediate products, modified silicone synthetic resin and mixturesthereof; 0.1 to 30% by weight of a heat stable oxidation catalyst; andan organic solvent.
 11. A cooking apparatus according to claim 10, andfurther including inorganic carrier particles for said oxidationcatalyst.
 12. A cooking apparatus according to claim 10, wherein saidself-cleaning coated film is formed by coating said coating compositiononto said oven wall and carbonizing said coating.
 13. A cookingapparatus according to claim 10, wherein said silicone synthetic resinis in the range of about 30 to 80 weight percent based on the weight ofsaid composition.
 14. A cooking apparatus according to claim 10, whereinsaid oxidation catalyst is in the range of about 0.1 to 50 weightpercent based on the weight of said composition.
 15. A cooking apparatusaccording to claim 10, wherein said organic solvent is in the range ofabout 5 to 30 weight percent based on the weight of said composition.16. A cooking apparatus according to claim 11, wherein said oxidationcatalyst is in the range of about 0.1 to 1 weight percent and saidcarrier particles are in the range of about 5 to 50 weight percent, bothbased on the weight of said composition.
 17. A cooking apparatusincluding an oven wall on which a self-cleaning coated film whichcatalytically oxidizes and removes fats and oils deposited thereon isformed, said self-cleaning coated film being formed from a coatingcomposition comprising: about 30 to 80 weight % of a silicone syntheticresin selected from the group consisting of straight silicone syntheticresin, silicone intermediate products, modified silicone synthetic resinand mixtures thereof; about 0.1 to 30 weight % of a thermostableoxidation catalyst; and an organic solvent.
 18. The cooking apparatus ofclaim 17, and further including carrier particles in an amount of about5 to 50 weight %.
 19. A cooking apparatus including an oven wall onwhich a self-cleaning coated film which catalytically oxidizes andremoves fats and oils deposited thereon is formed, said self-cleaningcoated film having a thickness of about 40 to 300 microns and beingformed from a coating composition comprising: about 30 to 80 weight % ofa silicone synthetic resin selected from the group consisting ofstraight silicone synthetic resin, silicone intermediate products,modified silicone synthetic resin and mixtures thereof; about 0.1 to 30weight % of a thermostable oxidation catalyst having an average particlediameter between about 10 and 200 microns; up to about 70% by weight ofcarrier particles for said oxidation catalyst having a diameter of about40 to 300 microns; and an organic solvent.
 20. A cooking apparatusincluding an oven wall on which a self-cleaning coated film whichcatalytically oxidizes and removes fats and oils deposited thereon isformed, said self-cleaning coated film comprising a self-cleaningcoating composition, comprising: a silicone synthetic resin selectedfrom the group consisting of straight silicone synthetic resin, siliconeintermediate products, modified silicone synthetic resin and mixturesthereof; 0.1 to 30% by weight of a thermostable metal oxidationcatalyst; and up to about 70% by weight of carrier particles which arelarger than said catalyst particles and which have a diameter of about40 to 300 microns.
 21. The cooking apparatus of claim 20, wherein saidcarrier particles are alumina.
 22. The cooking apparatus of claim 20,wherein said catalyst particles are a noble metal.
 23. The cookingapparatus of claim 22, wherein said noble metal is palladium.
 24. Thecooking apparatus of claim 20, wherein said carrier particles are analuminum compound.
 25. The cooking apparatus of claim 22, wherein saidcatalyst is present in an amount of 0.1 to 1% by weight.
 26. The cookingapparatus of claim 20, wherein said catalyst particles are coated onsaid carrier particles and said carrier particles are present in anamount of between about 30 and 70 weight %.
 27. The cooking apparatus ofclaim 20, wherein said carrier particles are present in an amount ofabout 5 to 50 weight %.
 28. The cooking apparatus of claim 20, whereinsaid catalyst is platinum.